My site looks different on different computers. How can I fix this? Is this related to screen resolution?

There is no standard rule in relation to screen resolution. Go to your home page editor to adjust screen resolution.

Most older screen resolutions are still using 800X600 pixels so you are best to type in 780 pixels to accompany for all machines (this will keep the scroll bar on the right in view as well), however, this may only represent up to 20% of the market. Alternatively, you can select to view the page as a percentage of the entire screen. Check if this looks good on your computer by changing your screen resolution between the highest and the lowest. Using a percentage view will allow the page to shrink when the customer narrows the window but can be stopped by a custom sized image such as your company logo or an image inserted in the center of the page.

It's best to test to determine what looks best for you. On different screens they may look different - test a few different percentages / pixels, save, and check the resolution on various computers.